The cut of a Diamond is the only property which is totally dependent on man. Although often overlooked, cut is actually one of the most important aspects to consider when choosing your diamond. A Diamond cutter analyzes the rough diamond, and has to determine how to extract the most beauty and most profit out of the rough stone.
Cut refers to not only the shape of the diamonds, but its proportions and finish, factors which determine the sparkle of the diamond.
The goal in terms of extracting the greatest beauty from a Diamond, is to have light enter a Diamond, disperse the light as it bounces inside the Diamond, thereby producing the different colors and sparkly effect, and finally returning as much light to the eye as possible.
According to conventional wisdom, the proportions shown above are the best for maximum light return. The 2-dimensional illustration below shows the theoretical path a ray of light will take through an ideal-cut Diamond. . As you can see, the rays of light entering the Diamond, reflect back to the eye. But it is possible for a diamond cutter to extract more weight out of the diamond by increasing the diameter of the stone. This will make the stone too shallow, and light may escape from the side of the stone, as shown here... or leak out of the bottom of the stone, like here... Another side-effect of this cut, is that it makes the stone appear larger. Don't be fooled by this. The fact that it appears larger than it is does not make it a better stone. If you compare a shallow stone to a well-cut stone, you will see the difference in how the well-cut stone "lights up".
By the same token, it is common
to see the opposite problem. A stone which is cut too shallow will "leak
out light" in much the same way as the shallow stone. Here are two
illustrations . . .
This is not to say that a shallow or deep stone is a sign of a poor, or "low-quality" diamond cutter. Sometimes the shape of the rough diamond makes it impractical to cut a stone closer to "ideal" proportions without losing significant weight. But it is important to note the "light leakage" which will result from this cut.
How much does "cut" affect cost?
The effect of all the different properties of Diamond on Cost is discussed in more detail in the Cost section. For cut, it can vary depending on the quality of the Diamond.